This invention relates, in general, to semiconductor devices and, in particular, to gate controlled transistors with high mobility. Mobility is the velocity acquired by charge carriers (electrons or holes) per unit strength of applied electric field. In semiconductors, normal carrier mobility ranges from 10.sup.2 to 10.sup.5 cm.sup.2 -V.sup.-1 sec.sup.-1. A materials conductivity is the product of the charge, the mobility and the carrier concentration.
A semiconductor in which the concentration of charge carriers is characteristic of the material itself rather than of the content of impurities and structural defects of the crystal is called an intrinsic semiconductor.
In an ideal intrinsic semiconductor, mobility is determined by lattice scattering; that is, collisions between lattice waves (phonons) and electron waves (electrons). In an actual intrinsic specimen there are always some impurity atoms which may dominate scattering at low temperatures when phonons are quiescent, but at higher temperatures lattice scattering, particularly by optical phonons, is dominant. At cryogenic temperatures (e.g., T=4.degree. to 77.degree. K.) ionized impurity scattering does indeed dominate mobility. In addition, the theory of Brooks and Herring predicts, and an experiment confirms, that as a result of electron-electron scattering at a given temperature, mobility decreases with increasing impurity concentration, and for each doping level there is a theoretical maximum mobility. Finally, it is known that, in general, the mobility of electrons (and hence n-type semiconductors) is greater than the mobility of holes (and hence p-type semiconductors).
A highly doped n-type semiconductor, therefore, typically suffers from low mobility both at low temperatures (e.g., 4.degree. K.) due to ionized-impurity scattering from donors used to dope the specimen, and at high temperatures (e.g., 300.degree. K.) due to electron-electron scattering and electron-phonon scattering. Thus the highest mobility semiconductors tend to be low doped so as to reduce both electron-electron scattering and ionizedimpurity scattering. But low doping levels cause commensurately low conductivity at room temperature due to a dearth of carriers and at cryogenic temperatures due to carrier freeze-out.
Consider the compound III-V semiconductor GaAs as an example. N-type GaAs samples typically exhibit room temperature mobilities of about 6,800 to 2,800 cm.sup.2 V.sup.-1 sec.sup.-1 for doping levels of 10.sup.15 to 10.sup.18 /cm.sup.3. But mobility is highly temperature dependent. A GaAs sample doped to 10.sup.17 /cm.sup.3 may have a mobility of several thousand at room temperature, but at helium temperatures the mobility may be less than a hundred. Extremely high mobilities in GaAs (e.g., 10.sup.5 cm.sup.2 V.sup.-1 sec.sup.-1) have been attained by vapor phase epitaxy in isolated cases by utilizing extremely low doped samples (e.g., 10.sup.13 /cm.sup.3). As mentioned previously, however, GaAs with such low doping levels suffers from low conductivity.
Dingle et al. in U.S. Pat. No. 4,163,237 disclose a GaAs seminconductor device with enhanced mobility achieved by fabricating the semiconductor in the form of relatively narrow bandgap (GaAs) semiconductor layers separated by wider bandgap (AlGaAs) semiconductor layers. The layers exhibit a conduction or valence band step sufficiently large to confine electrons or holes, respectively, to the narrow bandgap layers. In addition, adjacent narrow and wide bandgap layers are substantially lattice-matched so that the heterojunctions formed at the interfaces therebetween are substantially defect free. An essential feature is that the wider bandgap layers are doped such that the impurity concentration-thickness product therein is greater than the same product in the narrower bandgap layers. Preferably, the narrow bandgap layers are doped n-type to a level which satisfies the foregoing product criterion.
The effect of the multilayered structure is to produce a potential or "quantum" well into which carriers flow from the adjacent wide bandgap layers. The wide bandgap layers become depleted of carriers which accumulate in the narrow bandgap layers as the multilayered structure is being fabricated. Because the narrow bandgap layers are undoped or unintentionally doped, the number of ionized impurities therein is extremely small compared to the number of carriers which will accumulate therein as long as the wide bandgap layers are doped such that the impurity concentration-thickness product therein exceeds the same product in the narrow bandgap layers.
As a result, the carriers, which are confined to the narrow bandgap layers by the heterojunctions formed at the interfaces with the adjacent wide bandgap layers, experience relatively little scattering from ionized impurities. The multilayered structure as a whole exhibits generally higher mobilities than are attainable in bulk samples of the narrow bandgap semiconductor material.
Because the heterojunction barriers are not infinitely high in energy, there is a finite quantum-mechanical probability that carriers may penetrate a few Angstroms into the wide bandgap material where ionized impurities are present. Thus to further reduce ionized impurity scattering, and further enhance mobility, in the event that such carrier penetration should occur, the doping of the wide bandgap layers is terminated short of the heterojunctions so as to leave thin (e.g., 10-60 Angstorms) buffer zones.
Mimura in U.S. Pat. No. 4,424,525 extended the Dingle et al. concept to produce a high electron mobility transistor in the form of a single active heterojunction device. The heterojunction is formed between a pair of layers fabricated with two different semiconductors having different electron affinities i.e. GaAs and AlGaAs. The "electron affinity" of a material is the difference between the lowest allowed energy state of an electron in a vacuum and its lowest allowed energy state in the conduction band of the material. In Mimura, the semiconductor layer having the lower electron affinity is doped with an n-type impurity. Due to the difference in electron affinity, electrons contained in the semiconductor layer having the lower electron affinity are depleted and move to the semiconductor layer having the higher electron affinity.
The electrons accumulate in an extremely thin region close to the single heterojunction. These accumulated electrons provide a channel. The entire quantity of the electrons are confined in an extremely thin region with a thickness of several tens of Angstroms and are spatially separated from the doped n-type impurity atoms. This means the electrons suffer less from ionized-impurity scattering. Therefore, the mobility of the electrons is significantly improved particularly at cryogenic temperature at which the effect of ionized-impurity scattering becomes dominant in determining the electron mobility. On the other hand, the electron source region is depleted to some extent. When the thickness of the electron source region is selected to a proper magnitude, it is possible to make the electron source region entirely depleted. As a result, the electrons accumulated along the single heterojunction function as the only channel for the layer configuration consisting of an electron source region and a channel region. Accordingly, when one or more insulated gates or Schottky barrier gates together with a source and a drain are placed on the top surface of the layer configuration a Field Effect Transistor (FET) is produced with a path of electric current limited to the channel formed of the electrons accumulated along the single heterojunction.
High Electron Mobility Transistors of the type described above in Mimura are referred to by the acronym HEMT. They are also sometimes referred to by the acronym SDHT for Selectively Doped Heterojunction Transistor.
In more generalized terms the HEMT or SDHT uses a "potential" or "quantum" well for electron conduction. This potential well is created by the band-gap discontinuity in the conductive band between two epitaxially grown layers of different composition, electron affinity, and doping.
A typical HEMT structure comprises an active channel layer of high electron affinity material (GaAs), about 1 micron thick, which is formed on the top surface of a n-type doped or undoped buffer layer (GaAs) which is formed on a semi-insulating substrate (GaAs). On top of the active layer a thin (20-60 .ANG.) layer of undoped lower electron affinity material (AlGaAs), called the "setback" or "spacer" is grown. An n.sup.+ donor layer of lower electron affinity material (AlGaAs) is then grown over the spacer layer. A capping layer of highly doped GaAs is grown over this donor layer to passivate the AlGaAs and facilitate ohmic contact. Gate metallization is then applied over the channel region and ohmic contacts applied to the drain and source regions.
Electrons supplied from the dopant atoms in the higher electron affinity AlGaAs donor layer drop into a triangular "potential well" formed in the undoped higher electron affinity GaAs channel to form a two-dimensional electron gas region, sometimes referred to as a "2 DEG". See Chapter 4 Gallium Arsenide Technology, Howard W. Sams & Co., Inc. (1985) incorporated herein by reference, for a more complete state-of-the-art summary of SDHT or HEMT devices.
Several problems exist with respect to conventional planar HEMTs. Chief among these is the occurrence of substrate leakage current. In conventional HEMTs, voltage applied to a gate electrode controls the current flow from source to drain (I.sub.SD). Preferably, the pinch-off voltage, i.e., the gate voltage required to completely stop I.sub.SD, should be as low as possible. Unfortunately, some leakage current always occurs, since electrons, though driven deeper and deeper into the underlying HEMT substrate by higher and higher gate voltages, will always find a leakage path from the source to the drain. Consequently, a need exists for an HEMT with good pinch-off characteristics, i.e., low pinch-off voltage which would preferably totally eliminate substrate leakage current.